Maximum power transfer occurs when an alternating current (AC) generator output impedance matches a load impedance. For radio frequency (RF) applications this is especially important as RF generating devices are most efficient when operating at their design impedance. A subset of RF generating devices may be, for example but are not limited to, RF transmitters and power amplifiers, and a subset of RF loads may be antennas used for transmitting RF. Generally, RF transmitters operating in the high frequency (HF), very-high frequency (VHF), ultra-high frequency (UHF), and microwave bands require a well-defined load impedance, e.g., 50-ohm or 75-ohm, to efficiently operate into for maximum RF power transfer. However, most antennas are not 50 or 75 ohms except at some specific frequency when carefully designed and adjusted to that frequency and load impedance. Therefore, to match the load impedance of a transmitting antenna to the output impedance of a RF transmitter an antenna matching network or “antenna coupler” may be used between the transmitter and antenna when the transmitter and antenna must operate at several different frequencies.
Typically, an inductor-capacitor (L-C) matching network is required which may be configured as a T-network (FIG. 1), also known as a high pass filter, or an L-network (FIG. 2) which may be configured as a low pass filter. Another configuration of the L-network is the π-network (FIG. 3) which has a first capacitor on the source side and a second capacitor on the load side.
The T-network has the following issues: a) there may be multiple L-C matching solution combinations which may have significant component stresses due to high RF currents and/or voltages, b) a “correct” (best) matching solution may be difficult to find, and c) the T-network requires more components to provide the correct matching solutions. The L-network may require unusually large values of inductance and/or capacitance, and the π-network, like the T-network, requires more components to provide the correct matching solutions.
Present technology automatic antenna matching networks will do an iterative selection of capacitance and inductance values until a match between the RF load and RF source is found, e.g., lowest voltage-standing-wave-ratio (VSWR). This, however, may take seconds and many sequential relay operations to achieve.